Save All with untitled document changes current document & MRU order

[peterflynn]

1. Open two existing files and create two untitled files (call them A, B, U1 and U2)
2. Click to view the files in this order: A, U2, U1, B (so B is current and A is least recently opened)
3. Save All

Result:
U2 is current document. If you navigate with Ctrl+Tab, the MRU order is now U2, U1, B, A

Expected:
B remains current document. MRU order remains B, U1, U2, A

[peterflynn]

This is happening because the Save As implementation has an assumption that the file is the current document, which isn't true with untitled docs + Save All.

Not sure if it's major enough to fix this sprint... although I could vaguely picture maybe doing the #4484/#4479 fix in a way that covers this too (fyi @bchintx).

[peterflynn]

Actually, I'm going to withdraw this. I think the way PR #4514 fixes #4489 produces a nice, elegant as-designed rationale for the behavior here that should feel pretty sane to users. Closing unless/until someone else raises this as an issue.